Selecting the right maleimide PEG reagent can determine the success or failure of a bioconjugation project. The maleimide group provides fast, selective thiol coupling, while the PEG spacer adds solubility, reduces immunogenicity, and gives researchers precise control over molecular architecture. But with hundreds of maleimide-PEG products available, identifying the right one for your specific application — ADC construction, protein PEGylation, fluorescent labeling, or surface functionalization — requires understanding each reagent’s functional group pairing, PEG chain length, and intended workflow.
This article profiles ten maleimide PEG linkers that cover the most common use cases in modern bioconjugation research. All products listed are monodisperse, meaning they have a single, defined molecular weight — no polydispersity distributions to complicate your mass spec data or batch-to-batch reproducibility.
For background on maleimide chemistry fundamentals, see our comprehensive maleimide chemistry guide.
Comparison Table: Top 10 Maleimide-PEG Reagents at a Glance
| # | Product | PEG Units | Functional Groups | Primary Application |
|---|---|---|---|---|
| 1 | mPEG45-NH-Mal | 45 | Maleimide + methoxy cap | Protein PEGylation |
| 2 | Maleimide-NH-PEG45-CH₂CH₂COONHS Ester | 45 | Maleimide + NHS ester | Heterobifunctional crosslinking |
| 3 | Maleimide-PEG8-CH₂CH₂COOH | 8 | Maleimide + carboxylic acid | ADC/peptide conjugation |
| 4 | Mal-PEG8-Val-Cit-PAB-MMAE | 8 | Maleimide + cleavable payload | ADC payload delivery |
| 5 | DBCO-CONH-PEG44-Mal | 44 | Maleimide + DBCO | Dual orthogonal conjugation |
| 6 | DAPE-PEG₄₅-NH-Mal | 45 | Maleimide + DAPE lipid | LNP/liposome functionalization |
| 7 | Biotin-PEG6-NH-Mal | 6 | Maleimide + biotin | Affinity capture / pull-down |
| 8 | MC-Val-Cit-PAB-MMAF | 0 (non-PEG) | Maleimide + cleavable payload | ADC (non-cleavable exit) |
| 9 | Maleimide-PEG4-COOH | 4 | Maleimide + carboxylic acid | Short-spacer conjugation |
| 10 | Maleimide-PEG24-NHS | 24 | Maleimide + NHS ester | Mid-length crosslinker |
1. mPEG45-NH-Mal — The Workhorse for Thiol-Directed PEGylation
mPEG45-NH-Mal is PurePEG’s flagship maleimide PEGylation reagent. With 45 ethylene glycol units (~2 kDa molecular weight) capped with a methoxy group on one end and a maleimide on the other, it provides substantial hydrodynamic radius increase upon conjugation to a protein.
Key specifications: – PEG length: 45 units (monodisperse) – Functional groups: Maleimide (thiol-reactive) + mPEG (inert cap) – Molecular weight: ~2,070 Da
When to use it: Site-specific PEGylation of cysteine-bearing proteins or peptides to extend circulating half-life, improve solubility, or reduce immunogenicity. The 45-unit PEG chain provides meaningful steric shielding while remaining small enough to preserve biological activity for most protein targets.
Why it matters: Unlike polydisperse PEG-maleimide reagents, this monodisperse version gives a single mass addition, enabling clean characterization by MALDI-TOF or ESI-MS. You get one peak, not a Gaussian distribution.
2. Maleimide-NH-PEG45-CH₂CH₂COONHS Ester — Heterobifunctional Crosslinker
Maleimide-NH-PEG45-CH₂CH₂COONHS Ester combines two of the most commonly used bioconjugation chemistries — maleimide (thiol-reactive) and NHS ester (amine-reactive) — on a single, long PEG45 scaffold.
Key specifications: – PEG length: 45 units (monodisperse) – Functional groups: Maleimide + NHS ester – Reactivity: Sequential — react NHS with amine first (more hydrolytically labile), then maleimide with thiol
When to use it: Creating defined conjugates between an amine-bearing molecule (antibody lysines, modified oligonucleotides, amine-functionalized surfaces) and a thiol-bearing molecule (reduced antibody, cysteine-tagged protein). The long PEG45 spacer is especially valuable when the two conjugation partners need physical separation — for example, when linking a large protein to a surface without steric occlusion.
Why it matters: The long, monodisperse PEG spacer provides exceptional solubility and flexibility. Most commercial heterobifunctional crosslinkers use short spacers (4–12 atoms); this 45-unit PEG version is rare and addresses real formulation challenges in multi-component bioconjugates.
3. Maleimide-PEG8-CH₂CH₂COOH — Compact Acid-Functionalized Linker
Maleimide-PEG8-CH₂CH₂COOH is a shorter maleimide-PEG linker terminated with a carboxylic acid group. The COOH can be activated with EDC/NHS for subsequent amide bond formation with amines, or used directly for conjugation to amine-functionalized resins.
Key specifications: – PEG length: 8 units (monodisperse) – Functional groups: Maleimide + COOH – Molecular weight: ~530 Da
When to use it: Ideal for applications requiring a compact linker — ADC payloads where excessive PEG length would interfere with receptor binding or intracellular trafficking, peptide-drug conjugates, and building-block synthesis for more complex constructs. The COOH handle provides flexibility: activate it to form an amide bond, couple to an amine resin, or use EDC chemistry.
Why it matters: This product is cited in peer-reviewed literature (Bioz-tracked citations) and represents the acid-functionalized maleimide PEG architecture that PurePEG pioneered for FDA-approved ADC applications.
4. Mal-PEG8-Val-Cit-PAB-MMAE — ADC-Ready Payload Construct
Mal-PEG8-Val-Cit-PAB-MMAE is a complete linker-payload module: maleimide for antibody attachment, a PEG8 spacer for solubility, a valine-citrulline dipeptide for cathepsin B cleavage, a para-aminobenzylcarbamate (PAB) self-immolative spacer, and the potent antimitotic payload MMAE.
Key specifications: – PEG length: 8 units – Cleavable motif: Val-Cit-PAB (cathepsin B-sensitive) – Payload: MMAE (monomethyl auristatin E) – CAS: 2353409-69-3
When to use it: Direct conjugation to partially reduced antibody cysteines for ADC construction. This is an off-the-shelf, research-grade linker-payload module — add your antibody (after reduction), conjugate at pH 6.5–7.0, purify, and characterize. No linker assembly required.
Why it matters: The PEG8 spacer significantly improves the aqueous solubility of the hydrophobic MMAE payload, reducing aggregation during conjugation and improving the pharmacokinetic profile of the resulting ADC. This design addresses one of the primary challenges with early-generation ADCs that used non-PEGylated linkers.
For broader context on ADC linker strategies, see our ADC linker technology overview.
5. DBCO-CONH-PEG44-Mal — Dual Orthogonal Chemistry
DBCO-CONH-PEG44-Mal is a dual-reactive linker carrying both a maleimide (thiol-selective) and a DBCO (dibenzocyclooctyne, for strain-promoted azide-alkyne cycloaddition) connected by a 44-unit PEG chain.
Key specifications: – PEG length: 44 units (monodisperse) – Functional groups: Maleimide + DBCO – Chemistry: Thiol-maleimide + copper-free click chemistry
When to use it: When you need to connect a thiol-bearing molecule to an azide-bearing molecule through a long, flexible, hydrophilic linker. Common scenarios include: (1) connecting an antibody cysteine to an azide-functionalized payload or nanoparticle; (2) building tripartite conjugates where one molecule is attached via thiol-maleimide and another via click chemistry; (3) surface functionalization where both chemistries are needed.
Why it matters: Orthogonal chemistry on a single molecule enables sequential, controlled conjugation without protecting-group strategies. The two reactive groups don’t cross-react, allowing one to be conjugated in the presence of the other.
Looking for a specific maleimide-PEG combination not listed here? PurePEG carries over 1,400 monodisperse PEG reagents. Browse the full heterobifunctional PEG linker catalog or contact us at 1-888-331-8188.
6. DAPE-PEG₄₅-NH-Mal — Lipid-Anchored Maleimide for Liposome Functionalization
DAPE-PEG₄₅-NH-Mal is a lipid-PEG-maleimide construct designed for incorporating thiol-reactive handles into lipid bilayers. The DAPE (1,2-diacyl-sn-glycero-3-phosphoethanolamine) anchor inserts into lipid membranes, the PEG45 chain extends into the aqueous phase, and the maleimide is presented at the distal tip for conjugation.
Key specifications: – PEG length: 45 units – Lipid anchor: DAPE – SKU: 460145 – Application: Liposome/LNP surface functionalization
When to use it: Incorporating targeting ligands (antibodies, peptides, aptamers bearing free thiols) onto the surface of liposomes, lipid nanoparticles (LNPs), or other lipid-based drug delivery systems. The PEG45 spacer ensures the maleimide — and consequently the targeting ligand — extends beyond the PEG corona of the liposome for effective receptor engagement.
Why it matters: Targeted liposomal delivery requires the ligand to be accessible, not buried within the PEG brush layer. A 45-unit monodisperse PEG spacer provides predictable chain extension and consistent ligand presentation — something polydisperse PEG-lipids cannot guarantee.
7. Biotin-PEG6-NH-Mal — Thiol-Selective Biotinylation
Biotin-PEG6-NH-Mal is a compact biotinylation reagent that attaches biotin to any free thiol via maleimide chemistry. The PEG6 spacer reduces steric interference between the biotin and the labeled protein, improving streptavidin binding.
Key specifications: – PEG length: 6 units – Functional groups: Maleimide + biotin – Application: Cysteine-specific biotinylation
When to use it: Site-specific biotinylation of proteins at cysteine residues for streptavidin pull-down, ELISA, Western blot detection, or immobilization on streptavidin-coated surfaces. Unlike NHS-biotin (which labels multiple lysines), maleimide-biotin labels only exposed thiols — giving defined stoichiometry and preserving activity at lysine-rich binding interfaces.
Why it matters: The PEG6 spacer is long enough to present biotin accessibly to streptavidin (the streptavidin binding pocket is ~13 Å deep) while keeping the overall reagent compact.
8. MC-Val-Cit-PAB-MMAF — Classic ADC Linker-Payload
MC-Val-Cit-PAB-MMAF uses a maleimidocaproyl (MC) attachment with no PEG spacer, a Val-Cit cleavable dipeptide, and the charged payload MMAF (which has a terminal phenylalanine instead of MMAE’s norephedrine).
Key specifications: – Spacer: Maleimidocaproyl (6-carbon, no PEG) – Cleavable motif: Val-Cit-PAB – Payload: MMAF – CAS: 863971-17-9
When to use it: ADC research where the non-permeable MMAF payload is preferred. MMAF doesn’t cross cell membranes efficiently on its own, so it has a reduced bystander killing effect compared to MMAE. This makes it suitable for targets with high antigen expression where bystander toxicity is undesirable.
Why it matters: Included here as the “non-PEG reference” — comparing this to Mal-PEG8-Val-Cit-PAB-MMAE (#4) illustrates the impact of PEG spacer incorporation on solubility and pharmacokinetics.
9. Maleimide-PEG4-COOH — Short-Spacer Building Block
For applications where a minimal spacer is sufficient, Maleimide-PEG4-COOH provides a compact maleimide-acid building block. With only 4 ethylene glycol units, it adds minimal molecular weight while still providing some hydrophilicity and conformational flexibility.
Key specifications: – PEG length: 4 units – Functional groups: Maleimide + COOH – Molecular weight: ~350 Da
When to use it: Peptide chemistry, solid-phase synthesis intermediates, or any application where the conjugate’s overall size must be minimized. Also useful in PROTAC synthesis where the linker connecting the target-binding ligand and E3 ligase recruiter should be short.
10. Maleimide-PEG24-NHS — Mid-Length Heterobifunctional Crosslinker
For applications that need more spacer length than PEG4 but less than PEG45, a Maleimide-PEG24-NHS crosslinker offers a middle ground. The 24-unit PEG chain provides ~1.1 kDa of hydrophilic spacer.
Key specifications: – PEG length: 24 units – Functional groups: Maleimide + NHS ester – Application: Moderate-distance crosslinking
When to use it: When PEG8 is too short (insufficient solubility or spacing) and PEG45 is too long (excessive flexibility, molecular weight concerns). This is common in bispecific construct assembly and surface-tethered bioconjugate applications.
How to Choose the Right Maleimide-PEG Reagent
When selecting from these maleimide bioconjugation reagents, consider:
- What is your second functional group? If you need amine reactivity, choose a Mal-PEG-NHS ester. If you need click chemistry compatibility, select the DBCO-PEG-Mal. For PEGylation only, mPEG-Mal suffices.
- How long should the PEG spacer be? Short spacers (PEG4–PEG8) minimize molecular weight addition. Long spacers (PEG24–PEG45) maximize solubility and steric shielding. Match spacer length to your application.
- Do you need a cleavable linker? For ADC applications requiring intracellular drug release, select a Val-Cit-PAB construct. For stable, permanent conjugation, choose a non-cleavable architecture.
- Is monodispersity critical? For any application requiring precise analytical characterization (regulatory submissions, clinical-grade conjugates), monodisperse PEG is essential.
For more guidance on PEG linker selection, read our PEG linker selection guide and our overview of PEGylated linker applications in bioconjugation.
PurePEG offers over 1,400 monodisperse PEG reagents, including maleimide-functionalized products spanning PEG4 through PEG45. Explore our complete product catalog or request a custom synthesis quote at 1-888-331-8188.